Field of the Invention
The present invention relates to a microfluidic system and method for performing a flash separation of a reservoir fluid sample.
Description of Related Art
In the analysis of oilfield reservoir fluids, it is often desirable to determine the gas-oil ratio, the formation volume factor, the monophasic composition, or other such characteristics of a particular reservoir fluid. Typically, the gas-oil ratio is defined as the ratio of the volume of the equilibrium vapor phase to the volume of the equilibrium liquid phase converted to standard conditions. The formation volume factor of the oil is defined as the ratio of the volume of the reservoir fluid at reservoir conditions to the volume of the equilibrium liquid phase of the reservoir fluid at standard conditions. The formation volume factor of the gas is defined as the ratio of the volume of the reservoir fluid at reservoir conditions to the volume of the equilibrium gaseous phase of the reservoir fluid at standard conditions. These characteristics are typically determined using a “flash” system or apparatus. For example, the monophasic composition is determined via a mass balance of the equilibrium vapor and liquid phases resulting from a flash of the reservoir fluid to standard conditions.
Currently, most flash experiments are performed in oilfield fluid analysis laboratories, and sometimes performed directly at the wellsite during wellsite pressure-volume-temperature (PVT) analysis, or are part of specific wellsite tools. Flash apparatus can be divided in two distinct categories: dynamic flash systems and static flash systems. Dynamic flash systems maintain fluid pressure, e.g., reservoir fluid pressure, upstream of a metering valve while maintaining atmospheric conditions downstream of the metering valve. Generally in dynamic flash systems, a pump that drives a single-phase sample and the metering valve are operated manually, which induces variability related to the geometry of the system and the skill level of the operator of the system. Accordingly, conventional dynamic flash experiments are sensitive to the speed at which the experiment is conducted. The accuracy of such experiments generally corresponds to the operator's skill, in that the operator must “feel” the metering valve for the cracking pressure and be very careful not to discharge the oilfield fluid through the metering valve at too high a flow rate. In practice, there is a tendency for the operator to operate the pump and/or the metering valve in such a way that the oilfield fluid flows at too high a rate, resulting in inadequate mass transfer from the liquid phase to the gas phase or vice versa, and erroneous readings. Errors in determining the correct “feel” of the metering valve can lead to liquid carry-over, resulting in inaccuracies in the gas-to-liquid molar ratio or inaccuracies in other such measurements. Moreover, dynamic flash apparatuses typically do not have a gas circulation system and, therefore, the gas does not remain in contact with the liquid for a sufficient amount of time for thermodynamic equilibrium to be attained.
Static flash systems, which are generally used in the laboratory, employ methods wherein a full sample of oilfield fluid is flashed to atmospheric conditions, followed by circulating or bubbling the gaseous phase through the liquid phase until thermodynamic equilibrium is attained. Static flash techniques are generally accepted as being more reproducible, as they do not depend upon the system operator's skill or experiment conditions, such as speed of the experiment. However, static flash methodologies require sophisticated and bulky equipment, which increases cost and requires a large footprint not well suited for field use at the wellsite.
While there are devices for performing a flash separation of reservoir fluid that are well known in the art, considerable shortcomings remain.